Vinyl chloride resins have excellent mechanical and chemical properties and are used for various purposes since they can be formed into rigid or nonrigid products when used with plasticizers.
In particular, a polyvinyl chloride resin for paste applications, hereinafter referred to as the “polyvinyl chloride paste resin”, is generally composed of particles having a diameter of 0.1 to 70 μm and is often used in the form of a plastisol prepared by dispersing the resin dried by spray drying or the like into a plasticizer to provide flowability. The plastisol composed of the paste resin can be subjected to various molding processes such as spread coating, dip coating, rotary screen printing, and spray coating, and can be easily formed into a nonrigid product by heating after shaping. Accordingly, the plastisol composed of the paste resin is widely used for various applications such as floor covering and wall paper in building materials, underbody coating and sealer in automotive materials, tarpaulins, and gloves.
A vinyl chloride/vinyl acetate copolymer resin prepared by copolymerizing vinyl chloride with vinyl acetate has been widely used for making products under low-temperature-processing conditions. Whereas the vinyl chloride/vinyl acetate copolymer resin has superior plastisol gelation properties and superior tensile properties in low-temperature-processing conditions, storage stability of plastisols and heat storage stability of the products therefrom are unsatisfactory.
Besides the vinyl chloride/vinyl acetate copolymer resin, the following methods using a paste resin for low-temperature-processing have also been suggested.
(1) A polymer blending method, which comprises blending a vinyl chloride polymer with a vinyl polymer having a low glass transition temperature, can improve the gelation properties of the plastisol. However, the storage stability and the heat stability are notably poor, and a homogeneous resin phase is hardly attainable. Thus, the tensile properties under the low-temperature-processing conditions are often poor.
(2) A method which involves making a copolymer of a vinyl chloride monomer and a monomer of a vinyl polymer having a low glass transition temperature (Japanese Unexamined Patent Application Publication No. 63-23947) improves the gelation properties of the plastisol. However, due to the difference among monomers in the polymerization kinetics, homopolymers therefrom tend to be formed. Moreover, the presence of a low molecular weight homopolymer may degrade the properties such as the storage stability, heat stability, and tensile properties in low-temperature-processing conditions.
(3) In a method which comprises graft polymerizing a vinyl chloride monomer onto a vinyl polymer using a cross-linking agent, e.g., a polyfunctional monomer or the like (Japanese Unexamined Patent Application Publication No. 63-264654), the productivity must be significantly decreased in order for the cross-linking agent to fully work. Moreover, an unreacted cross-linking agent degrades the storage stability and heat stability.
In other words, the methods described in paragraphs (1) to (3) can improve the gelation properties of the plastisol and the tensile properties of the product under the low-temperature-processing conditions but suffer from problems such as poor storage stability of the plastisol and poor heat stability of the product.
A method that uses a pyrolytic organic foaming agent such as azodicarbonamide or oxybis(benzosulfony hydrazide) is commonly used to make nonrigid polyvinyl chloride foams. Polyvinyl chloride copolymer resins, however, have low melt viscoelasticity around the decomposition temperature of the foaming agent. Thus, coalescence of cells of the foams or inability to maintain thickness due to failure in keeping the generated gas (permanent set) may result.
On the other hand, it is widely known that graft copolymers with relatively highly controlled structures and compositions can be made using macromonomers.
However, as disclosed in Japanese Unexamined Patent Application Publication No. 4-173818 and U.S. Pat. No. 5,177,151, although application of such graft copolymers as an additive to a matrix resins is common, no example that uses a copolymer as the main component of paste resins has been reported.
Moreover, as disclosed in Japanese Unexamined Patent Application Publication No. 4-173818, although application of such graft copolymers from macromonomers into agents for modifying friction resistance of nonrigid polyvinyl chloride is known, no example that intentionally uses such graft copolymer as an additive for accelerating internal plasticization to improve modulus of elasticity and flowability are known. This is due to the difficulty of stably producing a graft copolymer from a macromonomer achieving good handling property for the graft copolymer which has a high compatibility with the polyvinyl chloride matrix resin, into which the graft copolymers are added thereafter.
On the other hand, use of paste resins as additives for rigid/nonrigid polyvinyl chlorides is widely known. However, the effect of improving flowability or promoting internal plasticization is small. Liquid plasticizers or different types of polymer may sometimes be added, but the effect is insufficient if added in small amounts.